The present invention relates to display control technology and more particularly to scroll technology for use in liquid crystal driving control, fluorescent tube driving control and the like as what is effectively applicable to liquid crystal control devices for displaying characters in a dot matrix form by utilizing, for example, character generator ROMs (Read Only Memories).
A liquid crystal display device utilizing a character generator as a display control form comprises a display RAM for storing character codes (hereinafter called "DDRAM"), a character generator RAM or ROM for storing character patterns such as character fonts (hereinafter called "CGRAM or CGROM"), a display address counter for reading DDRAM by adjusting the DDRAM to a position at which a liquid crystal display panel is driven, and a liquid crystal driving circuit for driving the liquid crystal display panel. In this case, the central processing unit (hereinafter called "CPU") writes character codes corresponding to characters to be displayed on the liquid crystal display panel to DDRAM. The display address counter reads the DDRAM sequentially in conformity with the position at which the liquid crystal display panel is driven and reads character patterns from CGRAM or CGROM with the character code thus read as part of an address. The character patterns sequentially read are then sent as liquid crystal lighting/nonlighting data to a shift register in the liquid crystal driving circuit, and all liquid crystal driving circuits simultaneously output lighting/nonlighting voltage levels at a point of time data on one line is stored so as to drive the liquid crystal display panel. The control operation above needs repeating as many as the number of lines of characters, line by line for display, because each character is made up of a plurality of lines in the vertical direction.
When the liquid crystal display device employing character codes is used to continuously scroll a plurality of characters on a display screen to the left or right, there are two methods that are considered feasible. One method is to scroll display characters by sequentially incrementing or decrementing the read start address of a display address counter for reading DDRAM to shift the reading position of DDRAM character by character to the left or right. Whereas the other one is to scroll characters by causing CPU to shift character codes within DRAM character by character to the left or right to rewrite the code. The present inventors examined these methods and have found out the following problems. In the case of the former method, a plurality of display lines on the display screen are simultaneously scrolled, though the load applied to the CPU is light. In the case of the latter method, all the character codes in DDRAM corresponding to a specific scroll display line need rewriting each time the scroll position is shifted from one character to another, though a specific display line may be scrolled selectively, and this makes the load applied to the CPU heavy. In the former and letter methods both, moreover, scrolling can be carried out only in characters and if it is attempted to scroll a plurality of characters continuously, the display characters will not move laterally and smoothly on the display screen, thus rendering the scroll display discrete and unnatural.
On the other hand, there is a bit map type liquid crystal display control device as another display control form. The display control device of this type is capable of visually providing smooth scrolling. More specifically, a liquid crystal display control device which is loaded with a bit map memory (hereinafter called "BPRAM") possessing lighting/nonlighting display data in pixels is used and CPU itself generates character patterns, directly writes the character pattern to the BPRAM, and rewrites the data stored in the BPRAM by shifting the data corresponding to a specific display line pixel by pixel to the left or right. Even in this case, however, the present inventors took notice of the fact that there had also arisen the following problems. In comparison with a liquid crystal display control device employing character codes, this liquid crystal display control device in question needs a large-capacity BPRAM, and also unavoidably causes the CPU to frequently rewrite the data in the BPRAM, thus making the load applied to the CPU extremely heavy. In order to carry out smooth scrolling, moreover, the liquid crystal display control device is of not practical use unless the CPU has high processing capability. As an example of a reference document describing display control technology with reference to character generator and bit map systems, there is "Microcomputer Handbook, p-171," published by Ohm Company on Dec. 25, 1985.